1. Field of the Invention
The present invention relates to a light-emitting element at least part of which includes an organic compound. The present invention also relates to a lighting device, a light-emitting device, and an electronic device which are provided with the light-emitting element.
2. Description of the Related Art
Development of a light-emitting device in which a layer containing an organic compound is provided between a pair of electrodes and light emission is obtained by current flowing between the pair of electrodes has been advanced. Such a light-emitting device is capable of being reduced in thickness and weight in comparison with a display device which is currently called a “thin-film display device”. Moreover, because of self-emission type, such a light-emitting device has a high level of visibility and high response speed. Therefore, such a light-emitting device has been actively developed as a next-generation display device, and has been partly put into practical use at present.
Such a light-emitting element can emit light of various colors depending on a material contained in a layer containing an organic compound, which serves as an emission center. Moreover, by stacking of layers containing an emission center substance which exhibits different emission colors, light emission overlap, and more variations of emission colors can be obtained. In particular, the emphasis is put on white light which can be obtained by overlapping of red light, green light, and blue light or overlapping of emission colors which are in a relationship of complementary colors because white light is suitable for the use of a backlight or lighting, in addition to a display.
Deterioration of a light-emitting element is given as one of the reasons why such a light-emitting device with many advantages is limited to a partial practical use. A light-emitting element is deteriorated such that luminance is lowered with accumulation of driving time even if the same amount of current is fed. In order to promote the light-emitting element, it is essential to obtain a light-emitting element whose degree of deterioration is acceptable for an actual product. A light-emitting element has been researched from many aspects such as aspects of a driver circuit, sealing, an element structure, and a material (for example, see Patent Document 1: Japanese Published Patent Application No. 2006-114796) and Patent Document 2: Japanese Published Patent Application No. 2007-220593).
However, actually, there are various causes of decrease in luminance with accumulation of driving time, and the present measures are not enough.